1. Field of the Invention
This invention relates to a method and apparatus for the nondestructive measurement of physical properties of a web or sheet, for example a sheet of paper.
2. Brief Description of the Prior Art
Paper manufactured for various applications and needs, must meet certain strength requirements. Such strength parameters are usually determined by tests which in the process destroys the sample being tested, for example, by the application of stress until the paper tears. This kind of destructive testing is obviously undesirable in some circumstances. For example, these tests may be performed on paper that has been wound on a reel. It must be cut from the reel and is usually conditioned to a standard moisture prior to being tested. This process requires a considerable time, which is undesirable for a test method used to control a paper machine since considerable off-quality product can be produced prior to detection and correction.
U.S. Pat. No. 4,291,577 described a method and apparatus for the nondestructive testing of paper as it is produced in a continuous fast moving web. The invention utilizes the known fact that many of the strength parameters of paper are related to Young's modulus of elasticity and the shear modulus. The moduli can be correlated with the velocity of sound waves traveling through the paper web. A transmitting transducer sends a mechanical signal to the paper and a receiving transducer picks up the ultrasound signal from the paper. By knowing the time it takes the ultrasonic waves to travel through the paper and the distance they travel the velocity of the ultrasonic waves can be calculated. The transducers are located in wheels which are in physical contact with the moving paper web. While the testing may be carried on in a nondestructive way while the paper is being produced, such a device is inherently complex. Thus, the wheels containing the sending and receiving transducers must be exactly synchronized. The sending transducer must contact the sheet and produce an acoustic wave which must be detected 1 to 100 microseconds later at some receiving transducer a short distance away. The receiving transducer must remain in contact with the sheet long enough so that it will not miss the first oscillation of the acoustic pulse which has been generated. These are serious impediments to the test method.
More importantly, the signal strength depends on the force with which mechanical sending and receiving transducers are applied to the moving web. This factor alone is a serious impediment to the use of the method, creating stress forces on both the apparatus and the moving web.
In addition, the physical properties of the paper web depend to a large extent on the elastic and shear moduli in the thickness or out-of-plane direction which present on-machine devices cannot measure.
Obviously it would be desirable to develop a method and apparatus which would test paper nondestructively, both in the in-plane and out-of-plane direction in the absence of physical contact or with minimal force applied to the web. The present invention answers the need for such apparatus by utilizing a laser beam for exciting the necessary acoustic signal in the web, thus eliminating one point of physical contact. The receiver can either be a mechanical transducer in contact with the paper or a microphone not in physical contact with the paper, thus minimizing, or in the case of a microphone, eliminating points of physical contact and stress.
The use of laser beams to generate acoustic waves is described in U.S. Pat. No. 4,169,662.